The invention relates to a steel for producing components for picture tubes, in particular for producing masks, shielding covers or frames for such picture tubes. Furthermore, the invention relates to a method for producing steel sheet intended for producing components for picture tubes, using such a steel.
Picture tubes of television sets, computer monitors and the like comprise so-called shadow masks, aperture masks or grille masks. These masks consist of a steel sheet, 0.05 to 0.2 mm in thickness, comprising a multitude of regularly arranged apertures or slots. Their function consists of delimiting the cathode ray of the picture tube before it reaches the coatings of the display screen, which coatings emit red, blue or green light. In this way an image of excellent sharpness and contrast can be generated on the display which is visible to the viewer. Furthermore, the masks shield the picture tube from external magnetic influences while at the same time preventing excessive electron radiation from reaching the surroundings.
To ensure effective shielding, as a rule the masks comprise a soft magnetic material based on a non-alloyed low-carbon steel enriched with Cr and killed with aluminium. Usually, such a steel is melted in a converter and poured to form slabs or thin slabs. These slabs are first hot rolled, then pickled, cold rolled for a first time before being subjected to a first annealing in decarburising medium, so as to achieve an extremely low carbon content. The strip annealed in this way is again cold rolled before the apertures or slots for the shadow masks are produced in a photoetching process.
Sheets are cut to size from the perforated starting material. In the case of slotted masks, these sheets are placed in a frame which keeps the thin stay-like slot delimitations taut by means of mechanical tension. In the case of perforated masks, if necessary, the sheets which have been cut to size, are subjected to recrystallising annealing before being deformed by deep drawing and stretch drawing. Both in the case of slot masks and in the case of aperture masks, final annealing is carried out during which the desired darkening of the mask is achieved.
This normal procedure is associated with disadvantages in that annealing in decarburising medium has to be carried out slowly, with an open coiler, in a bell-type annealing furnace. Furthermore, the rolling geometry during the second cold rolling step has to be maintained with the highest precision which means that the rolling mill used must meet the highest requirements concerning controllability and accuracy of dimension.
The production process and operational requirements of masks in picture tubes dictate that the starting material must have a good degree of purity and defined grain sizes so as to ensure a clean etched image. Furthermore, the material must have a yield point without permanent elongation so as to be able to be deformed without any danger of deformation lines occurring.
The material for tensioned masks must have good form stability also at the considerable tension forces acting upon it in the installed state, with such form stability having to be ensured even under the influence of heat during further production steps. Otherwise there is a danger that relaxation of the mask material will result in deformation of the apertures or slots made in the material. Such loss of tension as a result of excessive yielding of the material, as a result of which the respective component loses its shape, leads to colour shifts during the projection of the image on the display screen because the colour dots are no longer properly rendered. The aforesaid applies in particular in the case of masks comprising regularly arranged apertures of small diameter.
The Japanese published application JP 62249339 A discloses an extra-low carbon steel which is intended to meet the requirements stated. Apart from iron and unavoidable impurities, this steel comprises 0.0040-0.01% nitrogen, less than 0.03% C, less than 0.03% Si, and less than 0.1% Alacid-soluble (all details in weight %). A nitrogen content of at least 40 ppm prevents residual elongation during recrystallisation annealing. It has been found that the greater the nitrogen content, the smaller the relaxation. The Japanese published application therefore recommends a nitrogen content of 0.0055-0.0075 weight %. However, taking into account the usually applied process steps in the production of steel, such levels of nitrogen content are undesirable.
It is thus the object of the invention to provide a steel material suitable for producing components for picture tubes, said steel material comprising outstanding characteristics concerning the requirements to be met by such components. Furthermore, it is the object of the invention to provide a method for producing steel sheet intended for producing components for picture tubes, using such a steel.
From the point of view of the material, this object is met by a steel with the following composition (in weight %):
if necessary further additives with the remainder being iron and the usual impurities.
In spite of its low nitrogen content, a steel with such a composition comprises characteristics which make it eminently suitable for producing components for picture tubes. Surprisingly, it has been found that the slight tendency to relaxation in the deformed annealed conditionxe2x80x94which in the state of the art occurs only with an increased content of Nxe2x80x94can be achieved at a lower nitrogen content if at the same time a reduced carbon content is taken into account. The addition of molybdenum and tin further improve the creep characteristics in the iron solid-solution. As a result, the invention provides a steel which makes possible the production of components for picture tubes, in particular aperture masks or grille masks with effective reduction or prevention of relaxation of the material during the production process.
According to the invention, the carbon content is limited to max. 0.01 weight %, because carbide fractions are increased if the carbon content exceeds this value. Such an increase would result in reduced etchability of the steel. In addition, exceeding the maximum carbon content according to the invention would increase the quantity of dissolved carbon in the steel, with a resulting increase in the flow resistance and extension of the flow limit after final annealing. These would have a negative effect on the deformability of the sheet. The carbon content of a steel according to the invention should therefore be as low as possible. According to a preferred embodiment of the invention, the carbon content is therefore limited to less than 0.008 weight %.
The nitrogen content, due to its strong interstitial effect in combination with the equally interstitial effect of carbon, affects the creep resistance. Therefore, according to the invention, the nitrogen content is limited to values below 0.0035 weight %. In this context, a nitrogen content below 0.003 weight % is preferred because at such a low content, a further improved creep resistance is obtained. In addition, as a result of limiting the nitrogen content according to the invention, with respect to the presence of aluminium, an increase in the fraction of unfavourable AlN is prevented. As a result of AlN precipitation, the etchability of a sheet made from the respective steel is also limited, with its deformability being reduced.
Due to the reduction of the AlN fraction which is associated with lowering the aluminium content, and the resulting improvement of etchability, the steel with the composition according to the invention is particularly well suited to producing components of picture tubes. At the same time, as a result of the very substantial prevention of AlN, all the nitrogen is kept in solution (interstitial). Together with the carbon, which at a low carbon content is also present in solution, from the point of view of reducing elongation as a result of blocking of dislocations, nitrogen is more effective when compared to a situation where these elements would be present in a precipitated form (in the form of AlN). From the point of view of the required profile of characteristics (low elongation values), the combination of low aluminium content in combination with low nitrogen and carbon content is thus particularly favourable.
Manganese prevents hot brittleness which would otherwise be caused by the formation of FeS, in that sulphur in the form of MnS is precipitated. Therefore, according to the invention, at least 0.01 weight % of manganese is added to the steel. However, at a manganese content exceeding 0.7 weight %, the steel becomes too hard and its formability deteriorates due to the hardening of manganese present in solid solution. In the steel according to the invention, an optimal compromise between the negative and positive characteristics of manganese can be achieved if the manganese content isxe2x89xa70.4 weight % andxe2x89xa60.6 weight %, preferably 0.5 weight %.
Phosphorus is a substitutional alloy element with a considerable hardening effect in solid solution. However, excessive hardening of the steel has a negative effect on its deformability. For this reason, according to the invention, the P content is limited to values below 0.1 weight %.
Sulphur forms sulphide-like non-metallic inclusions which have a negative effect on the etchability of sheet made from such steel. For this reason, the S content according to the invention is less than 0.1 weight %.
During steel production, aluminium is added for deoxidisation. As mentioned, in conjunction with nitrogen, this can result in the formation of AlN. Apart from reducing the etchability, these aluminium nitrides also cause increased hardness of the steel. For this reason, according to the invention, the content of Alacid-soluble is limited to less than 0.008 weight %. The Al content should be as low as possible; in a preferred embodiment of the invention it is less than 0.003 weight %.
Chromium is added to a maximum content of 0.1 weight % because this element increases the adhesive property of the oxide film which is generated by darkening annealing as part of the production of masks. If more than 0.1 weight % of chromium is added to a steel of the type according to the invention, its hardness increases too much to still maintain the required deformability.
As an element with a large difference in atomic radius compared to iron, molybdenum enhances creep resistance and thus resistance to relaxation of the steel material during the production of components, in particular aperture masks or grille masks, for picture tubes. However, with a content exceeding 0.08 weight %, Mo excessively impedes recrystallisation during intermediate annealing which may be carried out. The influence of molybdenum on a steel according to the invention is optimal if the Mo content is less than 0.006 weight %.
Tin has the same positive effect on creep resistance as does molybdenum. At the same time however, tin causes the steel to become brittle. For this reason, according to the invention, the tin content is limited to values of less than 0.015 weight %.
According to a further advantageous embodiment of the invention, up to 0.3 weight % of tungsten is added to the steel. This addition of tungsten further improves creep resistance.
Due to its good form stability under considerable load, its good etchability and its magnetic properties, the steel according to the invention is eminently suitable for producing shadow masks, aperture masks or grille masks, frames or shielding covers for picture tubes.